This invention relates to tamper-resistant closable bags and envelopes and, more particularly, it concerns an improved closure sealing arrangement for envelopes of the type in which a band of pressure sensitive adhesive, initially covered by a releasable liner strip, is effective to seal opposing panel surfaces at the opening through which the envelope is filled.
Envelopes formed from pliable thermoplastic sheet materials have become increasingly popular for use by courier service organizations to package documents and other items to be shipped, usually on an expedited basis. Such envelopes are especially suited to this use because of the relatively high strength of the plastic sheet material, resistance to damage by water and other liquids, imperviousness to moisture generally and because of the ability of the sheet material to be printed in a wide range of distinguishing colors and styles. Additionally, the plastic sheet materials of such envelopes are receptive to a variety of pressure-sensitive adhesives by which a strong and secure sealed closure of the envelope may be effected after it is filled.
Although various thermoplastic envelopes and sealing methods are known, U.S. Pat. Nos. 5,045,040 and 4,932,791 disclose an especially useful envelope closure seal and method for envelopes having front and back panels joined along bottom and side edges and in which a release liner strip delimits overlying marginal edges along the top of the panels. The liner strip initially covers a pressure sensitive adhesive band in the top marginal edge of one of the panels and extends through the side edge junctures of the panels to enable removal of the liner strip and securement of the top margins by the pressure sensitive adhesive band. Registered lines of perforations are formed across the top portion of both panels below the bottom edge of the release liner strip to define a tear line for opening the envelope after it has been sealed in embodiments not intended for liquid contents. In envelopes which must be completely sealed, e.g., envelopes for containing a liquid, the lines of perforations are omitted and the adhesive band joins with fused side edges of the envelope.
Hot-melt, pressure-sensitive adhesives having an aggressive bond to the envelope material are typically used to effect closure of envelopes which are required to afford security to the package once it is sealed. A band, or bead, of an adhesive is usually applied in a molten state directly on the thermoplastic sheet material and then covered with a removable silicone-treated release liner that is removed at the time of use.
A problem that exists with present closure systems, however, is that when the closure is subjected to very low temperatures, e.g., by applying dry ice or spraying the closure with liquid nitrogen, Freon or another similar fluorocarbon or chlorofluorocarbon or other gas used for flash coolants, the closure can, for a brief time, be opened without any evidence of tampering and re-closed when ambient temperature is again reached, at which time the envelope returns to its original sealed condition. This is due to the fact that at extremely low temperatures adhesives of this type lose the ability to retain a bond to the material to which they have been adhered.
Attempts have been made to thwart such tampering and eliminate the violation of security packaging by the use of tamper-evident sealing systems for plastic envelopes. One such system is described in U.S. Pat. No. 5,077,001, which discloses the use of a tamper-evident sealing means having a pattern of regions having visibly distinct reflective characteristics and which provides visible evidence of any attempt to open and reseal the package. This patent discloses that such a tamper-evident system is needed because, due to the non-porous nature of plastics such as polyolefins, the adhesive does not penetrate into the plastic and only adheres to its outer surface, such that upon application of high or low temperatures the adhesion (between the adhesive and the plastic) dissipates or releases the bond as it temporarily recrystallizes, permitting the flap to be opened and then re-sealed as the adhesive regains its properties when it returns to normal temperatures.